Drop dependency on opencensus-ext-flask

Until https://github.com/census-instrumentation/opencensus-python/issues/1022
is resolved, this is locking us on a Flask 1 dependency, which can
no longer be built successfully (not easily).

ai_web_common/flask_ai/tracing.py

@@ -2,11 +2,6 @@ import json
 import os
 from urllib.parse import urlsplit
 from opencensus.trace import config_integration
-from opencensus.ext.zipkin.trace_exporter import ZipkinExporter
-from opencensus.ext.flask.flask_middleware import FlaskMiddleware
-from opencensus.trace.samplers import AlwaysOnSampler, AlwaysOffSampler, \
-    ProbabilitySampler
-from opencensus.trace.propagation.b3_format import B3FormatPropagator
 
 tracing_config_file = os.getenv('TRACING_CONFIG', '/etc/tracing/client.conf')
 
@@ -20,39 +15,6 @@ def setup_tracing(app, service_name):
     if 'report_url' not in config:
         return
 
-    # Configure the Zipkin exporter.
-    report_url = urlsplit(config['report_url'])
-    report_host, report_port = report_url.netloc.split(':')
-    trace_exporter = ZipkinExporter(
-        service_name=service_name,
-        host_name=report_host,
-        port=report_port,
-        endpoint=report_url.path,
-        protocol=report_url.scheme,
-    )
-
-    # Configure the sampler.
-    sample_pct = config.get('sample', 'always')
-    if sample_pct == 'always':
-        trace_sampler = AlwaysOnSampler()
-    elif sample_pct == 'never':
-        trace_sampler = AlwaysOffSampler()
-    else:
-        trace_sampler = ProbabilitySampler(float(sample_pct))
-
-    # We use the B3 (Zipkin) propagator because it seems to be the
-    # default in the Go implementation.
-    propagator = B3FormatPropagator()
-
-    # Set up the OpenCensus Flask middleware.
-    middleware = FlaskMiddleware(
-        app,
-        sampler=trace_sampler,
-        exporter=trace_exporter,
-        propagator=propagator,
-        excludelist_paths=['static'],
-    )
-
     # Patch the 'requests' module so that HTTP clients propagate the
     # trace ID. Note that this is mostly useless, as the requests
     # integration only patches the high-level API (get, post) and does

setup.py

@@ -16,7 +16,6 @@ setup(
         "flask-talisman",
         "opencensus",
         "opencensus-ext-zipkin",
-        "opencensus-ext-flask",
         "opencensus-ext-requests",
         "sso",
         "urllib3",